Filters may be employed with respect to sets. Insertion into a filter can be performed with respect to in-set entities. If it is subsequently desired to know whether or not an entity is a member of the set, a lookup can be performed with respect to the filter.
Certain filters allow for removal to be performed with respect to entities which cease to be in-set. However, filters which offer such ability tend demand that onerous sacrifices be made in return for this removal ability.